Ferredoxin from Halobacterium of the Dead Sea. Structural properties revealed by fluorescence techniques

The two tryptophan residues of ferredoxin from Halobacterium of the Dead Sea differ in their fluorescence characteristics. One of these tryptophan residues (class 1) absorbs more to the red and is thus probably in a more apolar environment than the other (class 2). Upon removal of the ferric ions, i.e., in the apoferredoxin, a 2.2-fold increase in the quantum yield of fluorescence is observed. A double exponential decay of the fluorescence is found for ferredoxin, reduced ferredoxin, as well as for the apoferredoxin. The longer decay time assumes a constant value of 6.9 ns in all three cases, indicating that it originates in a tryptophan residue which is not affected by changes in the Fe³⁺ binding site (class 2 tryptophan). The shorter decay component increases gradually from 0.55 ns in oxidized ferredoxin, through 0.80 ns in the reduced ferredoxin to 1.24 ns in the apoprotein. This decay component is thus assumed to be largely due to the second tryptophan residue of the protein (class 1) located close to the Fe³⁺ binding site. On the other hand, the relative decay amplitude of the class 2 tryptophan is doubled upon formation of apoferredoxin. It is concluded that the class 1 tryptophan is quenched by the active site ferric ions and that the class 2 tryptophan is partially exposed to a polar environment. Whereas class 1 tryptophan may be similar to the single nonfluorescent tryptophan of spinach ferredoxin, class 2 tryptophan is found in a peptide which is present only in halophilic ferredoxins. Conformational changes occur in the molecule upon removal—but not reduction—of the ferric ions, causing the environment of the class 2 tryptophan to become more hydrophobic. It is possible that the class 1 tryptophan is associated with the occurrence of a higher redox potential in this ferredoxin, when compared with chloroplast-type ferredoxins.